1. Technical Field
The present disclosure relates to a power supply for lamps, particularly for LED-MR16 lamps.
2. Description of the Related Art
The low voltage lamps, e.g., LED-MR16 (Multifaceted Reflector) lamps, are normally supplied with alternating current, typically a voltage of 12 VAC, for example, by means of traditional transformers or electronic transformers.
Halogen lamps represent a substantially resistive load for the electronic transformers, which have been designed to be cheap and compatible with classic phase slicing variators. The output signal of these transformers is basically a high frequency square wave, e.g., ranging between 30 kHz and 80 kHz, with a sinusoidal modulation in the envelope between 100 Hz and 120 Hz. Such a wave form is a consequence of the internal circuitry, essentially based on a self-oscillating circuit supplied by the rectified network.
The LED-MR16 lamps offer different advantages as compared to the traditional halogen lamps. These lamps should be able, where possible, to operate both in the presence of traditional transformers and in the presence of electronic transformers.
The LED-MR16 lamps often include an integrated switching circuit that, when supplied by a low-value rectified voltage signal, conveniently adjusts the current in the LEDs. FIG. 1 shows a block diagram of a typical circuit application for LED-MR16 lamps. The voltage signal Vtr from the electronic transformer is at the input of a diode bridge 1 adapted to rectify the voltage Vtr. The rectified voltage Vtr-ref is at the input of both the capacitor C and a switching DC-DC converter 2 supplying the LED-MR16 lamp 3. Therefore, the voltage signal from the transformer should be rectified and evened out so the integrated circuit of the lamp may use it. However, because low network frequencies are involved, typically 50 Hz or 60 Hz frequencies, a heavy filtering action on the pulsing component or ripple at 100 Hz-120 Hz of the voltage signal is required in order to avoid the voltage at the input of the integrated circuit from being lower than a minimum voltage threshold required for the correct operation thereof.
If the voltage at the input of the integrated circuit of the lamp is lower than the minimum threshold, however, the light on the LED lamp becomes flickering upon an excessive ripple on the input voltage due to the repeated switching-off; the so called “flickering” phenomenon occurs.
The electronic transformers, designed for a resistive load, show an even more penalizing behavior when used with LED-MR16 lamps.
Moreover, the presence of the capacitor C is the cause of an irregular operation of the electronic transformer leading to an even stronger pulsing component. FIG. 2 shows the voltage Vtr in FIG. 1 at the output of an electronic transformer and FIG. 3 shows the voltage Vtr-ref at the output of the diode bridge 1, where average value Avg=12.9V and ripple=6.9V.